Frequency modulation receiver



Nv. 22, 1949 M. s. coRRlNG-roN 2,488,585'

" ,FREQUENCY MODULATION RECEIVER Filed May 29., 1945 2 Sheets-Sheet 1 Q wr m- .Il QN@ VJ N s u Nov. 22, 1949 M. s. coRRlNGToN 2,488,585

FREQUENCY MODULATION RECEIVER Filed May 29, 1945 2 sheets-sheet 2 Wines/y Patented Nov. 22, 1949 FREQUENCY MODULATION ER Murlan S. Corrington, Camden, N. J.,`assignor ,to n n Radio Corporation of America, acorporation of Delaware Application May 29, 1945,'-Serial No. y596,474 Y My'present invention relates generally to frequen-cy modulation receivers utilizing locked-in oscillator circuits, and more particularly to novel means for extending the lock-in range of frequency dividers of the locked-in oscillator type and detecting the output thereof.

In my application Serial No. 513,371 led December 8, 1943, I have disclosed and claimed novel circuits for receiving angle-modulated carrier waves and employing a locked-in oscillator system acting as a frequency divider, These circuits Were improvements on the locked-in oscillator circuit disclosed and claimed by G. L. Beers in his U. S. Patent No. 2,356,201, granted August 22, 1944.

In the operation of the Beers circuit angle modulated waves of a predetermined mean'or center frequency F are applied to the input control grid of a tube which may be of the pentagrid type, the output or plate electrode circuit being tuned to a desired subharmonic F/n, where n is a small integer. In the illustrative embodiment described in the Beers patent, the th subharmonic is developed across the plate circuit, i. e., n=5. For some purposes it is desirable to extend the lock-in range of the circuit shown in Fig/1 of the Beers patent, While still retaining the advantages of a lock-in range limited to suit the requirements for best reception of the incoming signal.

Moreover, I believe the operation of the l 6 Claims. (Cl. Z50-20) locked-in oscillator shown in the Beers` patent As will be further described below, such injected Vfrequency will be higher or lower than the mean oscillator frequency as the incoming frequency varies above or below its mean frequency.

quency which controls the oscillator frequency.

An important object of my present invention is to provide a locked-in oscillator of Wide lock-in range whose constants are variable thereby to secure ready control thereof. To this end, my

invention facilitates the control of the oscillator frequency by the injected currents produced as the incoming frequency differs from its mean ,'frequency.

My aforesaid application provided one method of increasing the lock-in range of a locked-in os- 4 cillator of the` type shown in the aforesaid Beers patent. The method comprised tuning the os- -cillator plate circuit suiciently to one side of F/n so that it would readily lock-in on one end of the frequency swing, and tuningn'the oscillator grid Y 2 Y circuit sullciently on Vthe other side of the desired harmonic 4F Ol' to help lock-in the oscillator on the opposite end of the swing.

In my application Serial No. 563,379, led November 14, 1944, novv Patent No. 2,440,653, granted April 27, 1948, I provided a novel method, distinct from that disclosed in my aforesaid application, of increasing the lock-in range of a locked-in oscillator by adding an electrical network in parallel with the oscillator tuned output circuit, and adjusting the added electrical network so thatthe equivalent input capacity decreased at the proper rate to just keep the oscillator in tune throughout most of the operating range. The equivalent series capacity of the network n-'as chosen to vary'with frequency at the proper rate to keep the oscillator in tune fora vportion of the lock-in range. One suitable added electrical network was the Vdiscriminator input circuit of'an FM detector of the type shown by J. D. Reid in U. S. Patent No. 2,341,240, granted February '8, 1944,'Where the said discriminator was desired to'be shunted directly across the plate circuit of the locked-in oscillator. Hence, the design ofthe FMA detector in that case was restricted by the fact that itsfinput was a part ofthe Alocked-in oscillator. The FM detector oi the said Reid patent is highly desirable from thev viewpoint of linearity of detection. At the same time it is equally'desirable, especially in the servicing of FM receivers, to have the characteristics `of 'the locked-in` oscillator substantially independentof those of the FM discriminator. Y

It is, thereforean important object of mypres- `ent invention to,v provideV such substantial independency, while utilizring'aV highly linear FM detector and securing a relatively wide lock-in range for the `locked-in oscillator.

Another important object of my inventionis to provide an improved Reid type FM detector whose'load resistors'are arranged to provide control over the FM detection characteristic.

Still other objects of my invention are to im- 4yprove generally lthe efciency, reliability and op- ,eration range of locked-"in oscillators, and more especially to provide locked-in oscillators of e'x- .tended operation range which are economical in 'cojnstruction and assembly.

Stillotherfeatures of my invention will best VVbe understood` by reference to the following defiscriptiomltaken in connection with the drawings,

3 in which I have indicated diagrammatically several circuit organizations whereby my invention may be carried into effect.

In the drawings:

Fig. 1 shows the basic locked-in oscillator circuit of the aforesaid Beers patent;

Figs. 1a, 1b and 1c are employed to analyze the circuit action thereof.;

Fig. 2 shows schematically an yembodiment of my invention;

Fig. 3 shows the construction of the locked-in oscillator transformer;

Fig. 4 graphically shows ideal response curves for the tank circuit of the locked-in oscillator;

Fig. 5 shows the phase angle shifts corresponding to the curves of Fig. 4;

Fig. 6 shows a typical linear FM detection characteristic; and

Fig. 7 illustrates a modified detector.

Referring now to the accompanying drawings, wherein like reference numerals in the diierent figures designate similar circuit elements, reference is made to the circuit shown in Fig. 1 for an explanation of the fundamental mode of operation of a locked-in oscillator which is of the general type shown in the aforesaid Beers patent. The circuit comprises a tube l which may be of the pentagrid type, but may also be of many other types capable of use as an oscillator. Between the input grid 2 and the cathode 3 there is impressed high frequency energy of a predetermined frequency F. The plate 4 has connected in circuit therewith a resonant circuit 5 which is tuned to a subharmonic F/n of F, the symbol n denoting a small integer. By way of illustration, the fifth subharmonic may be employed. However, a 3:1 or 4:1 or other frequency division ratio may be used. The plate 4 is, of course, established at a positive direct current potential with respect to the grounded cathode. The second and fourth grids of the tube are connected in common to a source of positive direct current potential, and these grids function as a positive screen. grid for the intermediate grid 6. As shown the coil 'I' has its lower end connected to ground through the resistor 9, and the latter is bypassed for high f-requency currents by the condenser I0.

The grid 6 is regeneratively coupled, as at. l, to the plate circuit 5. The fth grid 3 of tube l is connected back to the cathode, and this grid, therefore, functions as a suppressor grid. The cathode 3, grid 6 and plate 4 provide the oscillator section of the circuit. Thisoscillator section produces oscillations of the Subharmonic frequency p These oscillations are continuously produced even in the absence of input energy atgrid 2. The oscillations developed across circuit 5 ma;l betransferred to any utilization network.

To aid in imparting a more ready understanding of my invention, I shall first state what I be lieve to be the best explanation of the manner of operation of the locked-in oscillator shown in Fig. 1. The current from the tube is not sinusoidal, but comes through as a series ofY pulses. These cause harmonics of the subharmonic frequency F/'nl to be developed in the plate circuit 5. These harmonics will be applied to the grid 6 because of the coupling 1. Furthermore, the grid '6 ls operating with self-bias, and draws grid current during the positive swings of voltage. These pulses, also contain harmonics of F/n.

Let us assume by Way of illustration that F has a value of 4300 kilocycles (kc.), which may be the intermediate frequency of a superheterof- -dyne FM broadcast receiver. This frequency will beat with the fourth (3440 kc.) and/or sixth (.5160

kc.) harmonics of the resonant frequency of the plate circuit thereby to provide the desired subharmonic of 860 kc. In this case it is assumed that n is equal to 5. When the fourth and sixth harmonics are present simultaneously, it can be shown that the result is a single injected current of variable amplitude and phase. The process is similar to that when only one harmonic is present. Usually the vfourth and sixth harmonics will be of unequal amplitude, and the eect of the weaker one is to produce relatively small variations in the other. When the frequency of the incoming signal is exactly five times the natural frequency of the oscillator, the harmonic difference component will be in phase with the current in the oscillating plate circuit. The circuit becomes stable in this condition, and the injected current will lock-in the incoming 4300 kc. signal with the 860 kc. current in the plate circuit. Since the injected current has the same phase and frequency as the normal current in the plate circuit itis. equivalent to an increased output from the tube.

Now assume for the moment that the incoming frequency is higher than 4300 kc. and is within the lock-in range, but that the oscillator has not yet locked-in. The effect of the fourth harmonic and the incoming frequency will be to inject s. current of slightly greater frequency than 860 kc. into the tank circuit. In Fig. la, OA is a vector assumed to be rotating 860,000 times per second, and represents the normal current in the oscillator tank circuit. Let AB be a vector representing the instantaneous injected current of frequency slightly greater than 860 kc., resulting from the fourth harmonic of the oscillator voltage beating with the incoming signal voltage. This vector will rotate slightly faster than 860,000 times per second, and thus will have an angular -velocity relative to OA equal to the difference of `the two angular velocities.

Consider, further, the instantaneous condition shown in Fig.- 1a. The injected current AB has a component AC, shown dotted, in phase with OA and another component AD (shown dotted) out of phase with respect to OA. The vector OB represents the resultant of OA and AB. Let this current OB be applied to a tuned circuit LC as shown in Fig. 1b. Since the circuit LC is tuned to 860 kc., it will be at resonance with respect to the current OC, which is also 860 kc., and equals a-l-ib. The quadrature current AD is a leading current at the instant shown in Fig. la, and the 'result is the same as though an additional con- 'd'enser C' is connected in the circuit LC. The effect is to decrease the natural frequency of the tunedcircuit LC.

Now consider the condition at a later instant, as shown by Fig. 1c, the incoming frequency being the same as in the preceding discussion of Fig. la.. The oscillator has not yet locked in. The vector AB has rotated to the new position as shown. The injected current AB now has an in-phase component AC as before, but the component AD is now lagging instead of leading. If the current OB is now impressed on the circuit of Fig. 1b, the lagging component AD will cancel part of the leading current through capacitor C, and this will be equivalent to reducing the capacity of C since it is now drawing a smaller leading current. This will raise ,the resonant frequency of the tuned circuit LC.

It is evident that the circuit of Fig. 1 behaves like a reactance tube circuit. It is easy to see that if the frequency of .the incoming signal is s approximately five times that of the frequency of the tuned circuit 5, a point will be reached when the frequency of the tuned circuit becomes exactly A'one-fifth of the incoming signal frequency. When this happens the oscillator will lock-in with the incoming signal. This means that the amplitude and phase of the plate current now remain xed with respect to the incoming signal.

If |the incoming signal is exactly five times the frequency of the tuned plate circuit, the vector AB willbe in phase with OA. As the incoming signal frequency is decreased the oscillator remaining llocked in with it, the vector AB rotates to some position such as that shown in Fig. 1a. A further decrease in frequency will rotate the Vector until it is 90 out of phase with respect to OA. Since this position gives the maximum amount of quadrature current it corresponds to the maximum amount that the oscillator frequency .can be pulled over, and thus gives th'e lower limit of the lock-in range. If the incoming signal frequency becomes greater than five times the plate circuit frequency, the conditions will be similar except that the Vector AB will be lagging, as shown by Fig. 1c, instead of leading. AThe upper limit of the lock-in range is reached when the injected current lags by 90. The lagging current tends to reduce `the effective capacity of the circuit, and thus raises the frequency.

'Ihe amount of fourth and/or sixth harmonic lcurrent which is applied to grid 6 is limited. This vlimits the lock-in range of the oscillator, since it limits the amount of the above-mentioned iniected current. The result is that when the deviation of the incoming signal from 4300 kc. exceedsapproximately 130 kc., the oscillator suddenly breaks out 'and the frequency goes back towards center (860 kc. in the example herein described). This means that the oscillator is no longer locked in; the ratio of the incoming frequency to the oscillator frequency is no longer a definite small integer. If the oscillator is being used as a secondary frequency standard, to compare two frequencies, to operate a clock or in an FM receiver, this break-out will be objectionable. The figure of m30 kc. as the approximate break-out point is for the conditions shown in Fig. 1, i. e. the oscillator is operating without Ia frequency discriminator or other -auxiliary circuit to extend the lock-in range and the grid vcircuit is not tune-d.

V.on both ends of .the frequency swing. Also, gradual drifts in frequency of the heterodyne oscillator and other circuits, due to, temperature or other changes d-uring operation, may cause mis- Vtuning and consequent break-out.

In accordance with my :present invention .the

ffbreak-out effect is substantially eliminated by extending the lock-in range of the oscillator. In

.accordance `with my present invention, and contradistinct from the methods of my aforesaid ap- .plications, there is provided an -auxiliary resonant network which is inductively coupled with Athe tuned plate circuitof the oscillator. 4This added, cr auxiliary, resonant network is adjusted in resonance so that it retards the phase shift of the plate current with changing frequency at the fproper rate to just keep the oscillator in tune throughout most of the operating range. This makes it possible to lock in the oscillator .over a greater frequency range for a given .amount of injected current. It should be noted that when a network is designed so that the oscillator. is in tune overa given band of frequencies, the reactance of the .oscillator circuit is zero over the same range.. This means that the phase-angle char.- acteristic (plate voltage with respect to plate current) is also zero in this range. Therefore, the functioning of th'e auxiliary resonant net-work coupled to tank circuit 5 can be explained in terms of the addition of a network so designed that the phase characteristic fof the combination is zero in the desired range.

In Fig. 2 I have shown a circuit embodying my inventionfor extension of the lock-in range of the oscillator section of -a frequency divider and ensuing discrimination of FM signals. 'Ilhe sysf tem shown in Fig. 2 includes the locked-in oscillatoracting as a frequency dividing network in a receiving system of the type disclosed inthe aforementioned Beers patent. The locked-in oscillator Ifunctions concurrently to reduc-e or divide the mean frequency of ythe applied FM signal waves, and proportionately to reduce the extent of frequency deviation of the waves. Assuming that the FM receiver is of .the superheterodyne type and that the networks lprior to the locked-in oscillator tube are 4conventional in nature, the transformer I3 will have its primary circuit I4 and its secondary circuit l5 each tuned to the l:operating intermediate frequency (I. F.) of the system.

The received FM Waves may be those which are transmitted in the present assigned FM band of 42-50 megacycles (mc.) or may be in the 84-102 mc. range proposed for the future. The invention is not limited to any particular frequency band or to the reception of FM waves, since it is generally applicable to angle modulated carrier waves. Those skilled in the art are fully aware of the fact that in each channel the FM waves transmitted in the aforementioned assigned FM band are presently allotted a maximum overall frequency swing of kc. with Yrespect to. the mean or center frequency Fc, Fig. 4. The frequency deviation at any instant vis dependent upon the amplitude of the modulation signals at the transmitter, While the time for a cycle of the frequency deviation is equal to the period of the modulating signal per se.

The collected FM waves are usually selected in one or more stages of tunable radio frequency amplification, after which they are combined with locally-produced oscillations at the first detector network. The output of the first detector or converter is the I. F. (intermediate frequency) energy. In other words, the I. F. energy is the FM wave Whose mean frequency has been reduced to a much lower frequency, but whose frequency deviation is unchanged. After amplification by one or more stages of I. F. amplifiers, the I. F. energy is applied to the locked-in oscillator for concurrent frequency division and reduction of the frequency deviation. The mean frequency of the I. F. amplifier is located at 4.3 mc., whereas the pass band of the network is more than 150 kcxwideV It is believed that a width of l200 kc. willi usually. provide suitable tolerances. 1- This -rangeof the locked-inoscillator.

signies: that. the'lnetfwonk IA; I5-is capable of eilicientlyv transmitting .the entire frequency swingsof the FM wave whose'mean frequency has been reduced. tothe operating. IX. E. value. WhilethevalueofA 413 mc; has.been assigned as the operating I.. F. value, it is to beclearly understood that any other Vsatisfactor-yvalue may he 4employed depending upon theyarious factors met with` in the design of the receiver..

The' FMI signal energy: with: its; meanzfrequency at the .operating I. value isl applied toathe input. gridiZ` of tubei I.. The lattermayibafor example, ai miniature .tube of the pent'agridtype. Theinput grid. 2 isconnected toJthe high alternatingipotentlal side of the secondary circuit I5. The.: low potential side off circuit I,5 isreturned to the grounded cathode 3: through a resistor R1 shunted by condenser C9,.the latter comprising .a network-designated by the numeral I6. The function of the network I6 is tc providevoltage acrossetheresistor elementR'i in response t'oagrid currentl flow through the input grid' circuit. Such grid? voltage developed across the resistor Rl. may be. utilized.'l for automatic volume control (AVC). The AVCS voltage is employed' automaticallytoi bias the control.' grids of.` thepreceding amplifier tubes. in' ai manner well-known to those skilledin theart.

The plate circuit 5i in Fig. 2Ihas.a pass.' band widthV of above-30=kc., while the meanV frequency rhas a valuelof' 860`kc. rIAhi's is appropriate since the effect. of the locked-inoscillator network has been .toA divide.' the meanffrequency of the FM wave energy by a. factor of; 5, and the overall frequency deviation-range has al'sol been divided by the sameffactor. Thepassbandwidth should besufficiently above 30y kc.v to provide suitable operatingitolerances. Further, the plate current is substantially constant regardless of amplitude variations at the signal input grid. Thisieffect is secured; as explained in the1 aforesaid- Beers patent, without the use of a special amplitude limiter stage. The advantages of frequency division at this point of the FM receiving system have` been explainedin the Beers-patent. The extension of the lock-in range of the oscillator accomplished by this invention ascompared with the arrangements shown in the Beers patent will enable reception` of waves which are frequency modulated over a wider range, and will guard against' the distortion which might otherwise oc'- -cur by reason of the aforementioned breakout effect. SinceY the FM- wave ener-gy devel'- oped across .the plate circuit 5 is' modulated in strict accordance with'. the originally received'- FM waves,. savel that the meanj frequency andi extent of frequency deviation haver been proportionately; reduced, the output of the lockedl-inloscillator may now be subjected to suitable-:discrimination andfrectication.

The discriminator. input circuit employed herein feeds a pairV of opposed diode rectiflers I'I- and I8. The discriminator network' is similar tol that disclosed' and` claimed by.- J..D. Reidin IJ. S. Patent No.Y 2,341,240; grantedzFe-bruary 8, l

1944. In accordance with my' present.. invention, the designi of the4V discriminator network isV ren'- dered. substantially independent. of the lock-in Further, the circuit has. been improved to provide. control. over -thedeg-ree- Vof* linearity of the' detection: charac.-

,teristic The opposed diodesv IfII and. I6? will be separately fed with signal energy which: hasf the form of. amplitude;- modulated wave energy by ylirtue .O fl .the action.l of the.: discriminator input 8 circuit which translates wave energy into corresponding amplitude modulated` carrier wave energy.

Returning,l now, to consider the speciic construction and operation of the locked-n oscillator circuit, the oscillator grid 6 of tube I is connected by lead 6' to the highv alternating potentialv side of tickler coil L1. The lower end of the coil is connected to ground: through the parallel-connected combination of resistor R3 and condenser C10. Condenser C2 shunts. the tickler coil L1. The grid circuit-5' is not necessarily resonatedl to a particular frequency and the capacity of C2 is not critical. In a practical circuit' the resonant frequency of L1C2 can be betweeny 1500 and 2000 kc. and the frequency of the 'tank circuit 5 can be 860 kc.

The screen grid 2 of tube I` is connected to a: source of positivepotentia-l +S, say volts, and is bypassed to ground for I. F. currents-by condenser C1. 'VI-he plate 4-of tube I isconnected `to the- -l-B terminal, say at volts through the resonant circuit 5. The resonant tank circuit 5 consists of the parallel-connected connl bination of coil Lz, resistor R4 and condenser C4. Thezresistor element may be made variable, as indicated by the arrow through the'resistcr, for" a reason to be explained at a later point. .Coils In and L2 are magneticallyy coupled to provide the proper amount ofV regenerative feed#- back in order continuously to produce the oscillations at the desired frequency division ratio. Condenser C5 bypasses the lower end of resisjtor R4 to ground for the oscillator current.

VThe auxiliary resonant network, which has been'referred to above, isdesignated 5.", and Corlsists of the parallel-connected combination of condenser C3, resistor R2 and coil La. The coil L; is magnetically coupled to the plate coil Le, and this coupling is indicated by the dashed arrow through both coils. The resistor Rz is preferably variable, as indicated. The degree of coupling between coilsv L3 and L2 predominantly determines the extent' of the lock-in range of the oscillator. There are other factors-which influence the'lock-inrange; but these factors are minor compared to the coupling between the aforesaid coils. The auxiliary resonant network 5 is tuned to'the'same resonant frequencyA as the plate circuit 5. For maximum lock-in range the auxiliary coupled circuit 5" is slightly overcoupled, (closer than critical coupling) with` respect to tank circuit 5.

In Fig. 3-1 have shown a method of` constructing` the locked-in oscillator transformer 1 of Fig. 2'so as to show the simplicity ofthe present arrangement for securing' an extended lock-in range. The coils L1, La and L3` are all mounted on a hollow coil form 20, which may be of the type generally used inI a standard' I. F. transformer designed to operate at 455 kc. The coil form is usually made of plastic.. phenolic resin, ror ceramic material, and is provided at either `end with the adjustable comminuted iron cores 2'I and 22. These iron cores asi is well known to those skilled in the art, are located within the bore of the form 20, and may be adjusted from either endl. of the coil form to vary the magnitude ofl inductance of each of the coils. Thecoil L3 is mounted so as to have-its inductive. magnitude varied by axial adjustmentsof iron Acore 2l, while the. coil Lz is. spaced from the coil La. The inductive magnitudeV of' coll 'La is varied .by axially.A adjusting theiron core -22.

Of course, the sections'of both coils La'and L2 will be so spaced as to provide the desired degree of coupling referred to above. The oscillator tank coil Lz may be composed of three pies, or sections, vin series-aiding connection.

The tickler coil L1Y may be formed of twopiesv," one on each side of coil L2, connected in series' relation. This is done to give fairly tight coupling without increasing the inductance value of coil L1 beyond the allowable limits. Of course,-

the coil L3 is, therefore, coupled to the tickler' ever, this increase will reach a point eventuallyy where the oscillator begins to give a distorted output in the middle of the lock-in range. Further increase in coupling will cause the oscilla-A tor to break-out in the middle of the rangel but not at the ends.V This provides a simple means f or adjusting the lock-in range. The same result can be obtained by means of the variable resistance R2, since a decrease in the Q of coil L3 is equivalent to' a decrease in coupling between L3 Vand L2. Loading a transformer with resistors decreases Vthe eiiective coupling between the windings of `the transformer, and, therefore, adjustment of the magnitude of either of the shunt resistors R2 or Re will provide a control overthe lock-in range.

In Fig. 4 I have graphicallyjportrayed response curves for the tankcircuit f the lockedin oscillator. The single-peakedcurve a shows the type of response curvesecur'ed when the auxiliary network 5" is removedfrom the system. or when the coupling between coils L3 and L2 is so small that `the lauxiliarycircuit does not aifect the response curve. Fig. 5'shows the corresponding phase angle characteristic of the plate voltage with respect to thegplate current.l In Fig. 5 the solid line curve a is the'phase angle characteristic corresponding toV thev response curve a of Fig. 4. It will be noted that the phase angle is zero at one point only, to wit: the fire-.-v quency Fc. Y 'f On the other hand when the auxiliaryv net-ry work 5 is inserted into the system, andiscoupled to the plate circuit to a degree sufficient to provide the dashed line double-peaked kcurve b of Fig. 4, there results the phase-angle characterstc b' depicted in Fig. 5. It will be seenV that the dashed line curve b' has a considerable extent of nearly zero phase angleA as the frequency is varied. VThe reactive plate current which must be supplied by the tube to lock in the oscillator is. therefore, small over a con'- siderable frequency range. In other words, the phase angle of the tank circuit impedance is` nearly zero over a considerable range of frequencies. This means that the circuit is almost resonant throughout this range, andY a small amount of reactive current from thev tube can cancel the slight reactance of the circuit to produce lock-in, thereby eifecting a corresponding increase in the extent or range of lock-in. x Y I have found that it is possible to secure a lockin range, through all prior radio .frequencyl and I. F. selectivity, of i130 kc.or more. To adjust' the lockedfinoscillator circuit, the tank circuit 5 is; tuned by adjusting core'22until the oscillatorlocks V in Y with an, 'incoming frequency. .modulated- Way signal from a signal generator. The latter may feed the signal into the I. F. channel at 4.3 mc.- The modulation is then removed from the car-v rier wave generated by the signal generator, leaving the ecarrier at 4.3 mc. The FM detector is then adjusted to zero balance by connecting a high-resistance voltmeter from either end of re-` sistor Rs to ground, and adjusting the inductance of the input coil L4 until there is no voltage from the output of the detector to ground. The coil L2 is then adjusted until the lock-in range is approximately symmetrical about frequency Fc. Coil I is then adjusted until circuit 5 is tuned' to the same frequency as 5. The lock-in range can be adjusted to the desired amount by either shifting the position of coil I e with respect to La, or by adjusting either resistor Rzor R4. A slight readjustment of coil L2 may be necessary for best results.

The curves of Fig. 4 and Fig 5 refer to the tankcircuit only, as seen by the plate of the tube.-

When the tube is added the curves for the tube plus the tank circuit differ from those shown, since the tube develops an equivalent shunt capacitance or inductance, in accordance with Figs.V

1a, b, c, which locks in the oscillator. My invention extends this lock-in range.

An ordinary volume control resistor can be used for either of resistors R2 or R4, and will give a smooth control of the lock-in range of the oscillator. Hence, this provides a simple means for obtaining adjustable selectivity in an FM re` ceiver, since varying the lock-in range 'results in variable selectivity. This property of the locked--fr in voscillator is Vdescribed in the aforementioned Beers patent. This variable selectivity, or'variable lock-in range, may also be secured by pro--l Viding ready adjustment of coil La with respect to L2. On the other hand, there could be provided an iron core that couples the two coils simultaneously.- In addition, it is within the scope of my invention to provide either separate or Vconcomitant adjustment of the resistance values of resistors R2 and R4 and the coupling between coils Le and L2.y

vThe maximum lock-in range occurs when coils L2 and L3 are slightly overcoupled. The dashed. line curve b of Fig. 4 indicates this condition of slight overcoupling. It can be shown that for maximum lock-in rangethe product Qmlc=1.1, where Qm is the geometrical mean of the Qv of coils defined by i M 2-1 LzLs where Mz-a is the mutual inductance between Lz and La. The lock-in range decreases 'uni'y formly as this product decreases from'1.1 to-vv wards zero.Y Hence, this explains'why variation of any of the three aforementioned components provides acontrol over the lock-in range.

Turning now to the FM detectorI utilize a simpliiiedy discriminator input circuit, and itis not necessary to design the discriminator input cir' cuit with any consideration for the extent of the lock-in range. This is obvious from what has preceded. The discriminator input circuitf'consists ofthe parallel-connected combination* of coil L4, resistor R5 and condenser C7. The upper end of the combination of elements is connected to the ranode 30 of diode I1, while the lower end ofzthe combination is connected to the anode 3lv of ithe opposed diode rlz. VThe high potentialiside ofgplate circuit is coupled to the anode 3D by condenser C6. The cathode '132 and cathode :33 of `diodesll and lrespectively are connected'by condenser .iCaand the cathode A33 'is grounded.'

Theresistor Rs isconnected tothe cathode v32, and is connected by the outputllead to the following audio frequency amplifier (not shown). A further condenser `29 may shunt resistor Rs and condenser. Cri-to, de-accentuate the. high. audio frequencies.

The load resistor Re is connected Vdirectly in shunt with the `space current path of diode 'l1 between the anode SO-and. cathode'?. The'load resistor 'R7 Vis connected similarly directly between a-node `3! andcathode 33. This manner ofconnecting the fload resistors of the detector circuitdiiers lfrom that shown in the aforesaid Reid patent, and provides simplification of the detector circuit. vIn general, the present FM detector circuit `functions in the .manner described in detail in the aforesaid Reid patent, and Y,it :is not necessary to go into detail with regardito such operation. vIt is believed suiiicient forf the purposes of the presentapplication to explain -thatfthe'tuned input, or discriminator-,section 40, together .with the inherent capacity of diode I functions to providethe discrimination action.

In Fig. :6I have depicted a .typical detection characteristic for thetype of detector shown in Fig. 2. The frequencies .F1 and F2 denote the opposite peak limiting -rfrequencies VThe resultant instantaneous output voltage of the detector is zerorat the frequencyI Fc. It is highly desirable, of course, that the'curvebe highly linear-between the-peak limiting-frequencies. The detector circuitshownin Fig. 2p0ssesses such a characteristic,.particularly when'theinputcircuitJMl is connected directly in shunt across lthe oscillator plate. circuit 5. Hence, the discriminator is highly desirable when `used in connection with an FM receiverof the type shown in Fig. 2. The resonant circuit dll is tunedto a frequency such that it possesses a parallel resonance near one of -the peak limiting frequencies, -Fz of Fig. 16, while it is series resonant with the vinherent anode to cathode capacityof diode |18 at'the other limiting frequency F1.

The resistorRs `is-eifectively in shunt with the entire circuit and controls the parallel Q of the discriminator circuit, while the resistor R'zis effectively-in shunt withthe capacity of diode I8 and controls the effective series Q of the discriminator circuit. For optimum linearity these two Qs should be approximately equal, and, therefore, the resistive magnitude of resistor R7 will not-necessarly edual that of resistor .R. It

ishighly desirable Vthatthe detection character-v istic be linear between peak limitingr frequencies as far apart as 30 kc.. since thatis the normal pass band required Ito be handled. Moreover. it is expected that the lock-in rangeshall be i130 koor more, and the -vfrequency spacing betwen the Vlimiting frequencies F1 and F2 should be substantially greaterthan one-fitth of the over--alllockin range of the locked-in oscillator for a five-toone divisionratio.

Each of the opposed rectiflers I1 and t8 includes-in circuit therewith its respective load resistor. At the instant when the applied signal energy has the-mean frequency value Fatherectified voltages across the resistors Re and Rv are equal in magnitude and opposite .in polarity.

spectivelyupontheextent and direction of frequencyfdeviationwith -respect to the-mean frequency. Accordingly the voltage takenoff from theseriesf-connectedfoutputloads of the opposed rectiers k'I1 .and fl .corresponds to the original modulation signal voltage applied to the carrier at the 4ltransmitter.

.Theaesistor Rs and-the condenser-#Z9 rto ground prorides afdevemphasisfnetwor-k whose function is well known in the art of frequency modulation reoeption. 'Briey the de-emphasis network acts todiminishtheresponse .at the higher audiofrequencies, since during .transmission of the -FM waves such .higher audio Afrequency Acomponents may, have rbeendisproportionately emphasized.

l.following list of illustrative circuit constaats issuppliedfor apractical embodiment-of the rcircuitfshown .in Fig 2.

Diodes l1, .|,8=6H6.tube B1=22 ,000 ohms R2,=2,50,000 ohms (maximumyalue) ,Ra;20,000 ohms B4=500,000 ohms (maximum value) R5.=.100,00,0 ohms Re.=100,000 ohms fFue-50,000 ohms Rs=10Q,000 ohms .Graal lrnicrofarad (mid.) .Cz .=,4 7 micro-microfarads (mmf.)

.,C.=27 -Ilm 04:33 mmf. ,Ctr-.0.1 .mfd .C.=33 mmf. .,C,7:-22 mmf. -,Cs=56 mmf.

4Cfs=220 mmf. C1o.= 400 mmf. Lia-,0,20 millihenry (mh.) @20.4.6 mh. yliar-1.00 mh. L4=0.96 mh.

l'Mutualinductance between c oilsq L1 and L2=0.20 mh.

The frequency discriminator described in Fig. 2 isa very desirable circuit for use withmyinvention, b ut it is not essential to the operation of the locked-in oscillator. Othercircuitscan be employed to give satisfactory results. For example, either of the detectors of the typeshown inthe 'aforesaid Beers patent may.be.used. Like.- wise, the circuit shown by S. W. Seeley in U. S. Patent No. 2,121,103 granted June 21, .1938, or the one disclosed by Conrad in U. S. .Patent No. 2,057,640 granted October 13, 1936may be em.- ployed. Furthermore, the discriminator input circuit'need not necessarily be of the balanced type, but may consist of a single tuned circuitso adjusted -that the center frequencyof the output from the locked-in oscillator falls in the middle portion of -the nearly straight portion, on either side of the resonance curve.

-The discriminator input circuit of Fig. 2 can, also,be modified to operate with a single diode as shown -by=Fig. 7. The resistor 35 in series with condenser C5 is used to decrease the detuning` effect-of the discriminator circuit on the lockedin oscillator. Capacitor 36 is chosen so as to be equivalent to the anode to cathode capacity of omitted diode I8 ofFig. 2. The capacitor 36 is connected from thelow potential side of circuit 40 to ground. The rectified signal voltage is taken off across load resistor R6. The audio signal output connection including resistor Rs is made to the anode end of the load resistor. The discriminator circuit 40, 36 functions in the same manner as described in connection with the discriminator circuit of Fig. 2.

While I have indicated and described several systems for carrying my invention into effect, it will be apparent to one skilled in the art that my invention is by no means limited to the particular organizations shown and described, but that many modifications may be made without departing from the scope of my invention.

What I claim is:

1. In combination with a tube provided with at least an electron emitter, a control element, a grid and output electrode, means for applying to the control element frequency modulated input current of a desired carrier frequency, a resonant tank circuit reactively coupling said grid and output electrode and tuned to a desired subharmonic frequency of the input frequency, a resonant network magnetically coupled with the tank circuit, said network having its frequency equal to the subharmonic frequency, and said coupling being adjusted to provide a relatively wide lock-in range for subharmonic frequency oscillations.

`2. In combination with a locked-in oscillator tube provided with at least an electron emitter, a control element, an oscillator grid and oscillator output electrode, means for applying to the control element high frequency current which is frequency modulated and has a desired mean frequency, a resonant tank circuit coupling said oscillator grid and output electrode and tuned normally to a desired subharmonic frequency of said mean frequency, a selective network tuned to la frequency equal to said subharmonic frequency, said selective network comprising inductance, capacity and resistance in shunt relation, means coupling said selective network to said tank circuit to a degree sufficient to substantially extend the lock-in range, and means for varying the magnitude of said resistance thereby to control the effective degree of coupling to the tank circuit and hence the lock-in range.

3. In combination with a locked-in oscillator tube provided with at least an electron emitter, a control element, an oscillator grid and oscillator Aoutput electrode, means for applying to the con-l trol element high frequency current which is angle modulated and has a desired mean frequency, a resonant tank circuit reactively coupling said oscillator grid and output electrode and tuned normally to a desired subharmonic frequency of said mean frequency, a selective network tuned to a frequency substantially equal to said subharmonic frequency, means coupling said selective modulated and has :a desired mean frequency, a resonant tank circuit reactively coupling said oscillator grid and output electrode and tuned normally to a desired subharmonic frequency of said mean frequency, and the improvement comprising a selective network tuned to .a frequency substantially equal to said subharmonic frequency, means coupling said selective network magnetically to said tank circuit, and means for varying the mutual inductance between the tank circuit coil and a coil in the seelctive network to control the lock-in range.

5. In combination, a locked-in oscillator tube provided with at least an electron emitter, a control element, an oscillator grid and oscillator output electrode, means for :applying to the control element high frequency current which is angle modulated and has a desired mean frequency, a resonant tank circuit reactively coupling said 0S- cillator grid and output electrode and tuned normally to a desired subharmonic frequency of said mean frequency, the improvement comprising a selective network consisting of inductance, capacitance and resistance in series which is tuned to a frequency substantially equal to said subharmonic frequency, means coupling said selective network to said tank circuit, and means for varying the resistance of said selective network to control the degree ofV coupling and hence the lock-in range.

6. In combination with a locked-in oscillator tube provided with at least an electron emitter, a control element, lan oscillator grid and oscillator output electrode, means for applying to the control element high frequency current which is angle modulated and has a desired mean frequency, a resonant tank circuit reactively coupling said oscillator grid and output electrode and tuned normally to a desired subharmonic frequency of said mean frequency, a. selective network tuned to a frequency substantially equal to said subharmonic frequency, means coupling said selective network to said tank circuit to increase the lock-in range, detection means in shunt with said tank circuit which is linear and substantially without effect on the lock-in range, and means for adjusting the Qs of said tank circuit and said selective network separately to give continuous variation of the lock-in range whereby the lock-in range can be restricted to include the whole of the range of frequencies of the desired modulated wave but not enough to include frequencies substantially outside of said last-mentioned range, so that interference due to noise frequencies or adjacent-channel frequencies is reduced.

MURLAN S. CORRINGTON.

REFERENCE S CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,273,144 Van Roberts Feb. 17, 1942 2,341,240 Reid Feb. 8, 1944 2,356,201 Beers Aug. 22, 1944 

